The invention relates to a method of and to an apparatus for copying originals, especially diapositives. More particularly, the invention relates to improvements in methods of and in apparatus for making reproductions of originals which are scanned, line-by-line, by photoelectronic means and whose images are reproduced onto photo-sensitive material, line-by-line or point-by-point.
Electronic scanning and reproducing techniques of the above outlined character are gaining in importance in spite of the fact that copying times are longer than those in accordance with conventional integral exposure techniques. The reason is that methods and apparatus which rely on line-by-line scanning of originals and on line-by-line or point-by-point reproduction render it possible to influence the copies, e.g., by electronically improving the image and/or by effecting a partial color correction. Reference may be had, for example, to German Offenlegungsschrift No. 30 04 717.
As a rule, the images of negative originals are reproduced on negative copying material, and the images of diapositives are reproduced on color reversal material, i.e., a material which becomes practically white in response to impingement of a large quantity of copying light but remains black in regions which are not exposed to copying light. Thus, low-density portions of the original must be scanned with large quantities of light, i.e., with light of high intensity. This creates problems in connection with radiation which is supplied by a CRT or a laser because beams which issue from such sources do not have a uniform intensity across the entire cross-sectional area but rather a substantially Gaussian intensity distribution. The diameter of the beam increases in response to increasing intensity to bring about a darkening or brightening, depending on the nature of the copying paper.
Extensive tests indicate that medium- and low-density areas prevail in a great majority of originals irrespective of the motif or theme, and that such areas contribute primarily or largely to the composition of the image. Thus, conventional techniques involve a reproduction of the most important portions of diapositives by a beam of high intensity and relatively large diameter, i.e., the resolution of the corresponding portions of the copy is less satisfactory than that of copy portions exhibiting a pronounced density. Consequently, when the image of a diapositive is reproduced on reversal material, either point-by-point or line-by-line, the subjective quality of the images is less satisfactory than that of images which are obtained with the same equipment as a result of reproduction of negative originals on negative photosensitive material.